The use of aluminum conductor wire in electrical circuits has many advantages over previously used copper conductors. An aluminum conductor is significantly lower in cost and substantially lighter in weight than copper conductors. The difficulty with aluminum conductors is providing a reliable means of terminating or interconnecting the conductor. Aluminum will cold flow over time; cold flow being the permanent deformation of the material under cold (i.e., non-elevated) temperatures. As a result, the cold flow of the aluminum interconnect will loosen a mechanical connection such as a crimp or screw-type terminal. The loosened connection increases the joint resistance which, in turn, generates heat that can accelerate the cold flow of the aluminum joint, causing a cycle of more heat, and more cold flow of the aluminum. Moisture can also enter the joint between the aluminum conductor and the typically non-aluminum terminal connection and start galvanic corrosion between the dissimilar metals and ultimately the terminal connection can fail. This, in turn, raises the overall cost of aluminum as a conductor since the conductor either needs to be re-terminated, or a completely new conductor needs to be used.
Ultrasonic welding has proven to be a reliable and permanent method of welding aluminum to copper, for example, and can be used to solve the termination problems with aluminum cables. The difficulty with ultrasonic welding of aluminum is the tendency of aluminum to eventually stick to the ultrasonic tooling. Using a standard ultrasonic welding system, the aluminum from the aluminum conductor eventually accumulates onto the welding tool. This degrades any future welds using the ultrasonic tool until the aluminum can be removed. In many cases, the aluminum cannot be removed, and a brand new ultrasonic welding tip must be introduced to continue the welding process. This can cause a significant increase in the expense of ultrasonic welding, along with a significant loss of welding time when the machine is down for repair or replacement.
U.S. Pat. No. 6,476,324 attempts to overcome this problem by pinching the conductor, covered with resin-made covers, and pinching it together between resinous chips. The wires are then pressurized, heated, and ultrasonically welded. The result is the resin bonding with the resinous chips. This solution, however, prevents a seamless conduction between the conductor and the terminal as there is a resinous layer in between. Furthermore, the conductor needs to be pre-coated with the resin, adding both time and materials prior to the welding process.
U.S. Pat. No. 4,545,519 attempts to overcome this problem by providing a shim material between the welding tip and the work piece to be ultrasonically welded. The shim material is made from a relatively hard material, creating a very low tensile strength bonding between the work piece and the shim material. After the ultrasonic weld, the shim material is broken off and separated from the work piece. This solution, however, adds extra time, and wastes material for each weld. After each weld, the system must physically remove the shim material from the work piece. Furthermore, while the tensile strength bonding between the shim material and the work piece is very low, there is still the potential for the removal of the shim material to either leave behind some shim material, or damage the conductor in the process.
What is desired, therefore, is a system and method of ultrasonically welding an aluminum wire to a terminal that prevents the aluminum from the aluminum conductor from accumulating on the ultrasonic welding tool, is inexpensive, and does not interfere with the welding process.